There is a fast development towards higher and higher data-rates in cellular communication systems. To achieve this, a number of different techniques are employed. One is to use higher order modulation, carrying more bits per second per Hertz of bandwidth. Another is to use multiple antennas (MIMO), transmitting and receiving multiple data-streams simultaneously at the same carrier frequency. The most important, however, is to use a wider bandwidth of the signal. There has been a dramatic increase of 100 times in bandwidth comparing Third Generation (3G) Long Term Evolution (3G LTE) (20 MHz) to Global System for Mobile Communication (GSM) (200 kHz). This, however, is not enough, and a next step will be to aggregate several LTE release 8 signals, to achieve a total bandwidth of up to 100 MHz for LTE release 10.
The problem of down-converting multiple component carriers can be solved in one of two ways: one direct-conversion receiver per component carrier or a heterodyne receiver with one IF to baseband conversion for each component carrier.
One direct-conversion receiver per component carrier is expensive in terms of both area and power consumption. From an implementation point of view, it is also very difficult to have several VCOs with frequencies close to each other operating simultaneously. This due to the interference and frequency pulling they are causing to each other.
A heterodyne receiver with one IF to baseband conversion per component carrier is subject to harmonic down-conversion unless the frequency translation is performed by an analog multiplier and the local oscillator (LO) is a pure sine wave. Since the IF LO most likely is available as a square wave, the odd harmonics will cause harmonic down-conversion.
To solve this problem, U.S. Pat. No. 7,509,110 suggests a mixer circuit that includes five component mixers connected in parallel. Each component mixer uses a phase-shifted version of the local oscillator signal for frequency translation to produce a component output signal from the input signal. The component output signals are scaled according to corresponding gain factors and combined to form the output signal. Thus this solution intends to resemble or imitate a sine/cosine waveform as an effective local oscillator signal to reduce the harmonic down-conversion, and to achieve this, a set of five mixers in parallel is required in order to reduce the impact of odd order harmonics within the first decade from the fundamental frequency.
Therefore, it is an object of embodiments of the invention to provide a method to avoid harmful harmonic down-conversion while still using an IF local oscillator with large harmonic content, such as a square wave local oscillator, and at the same time reducing the complexity of the mixing circuit.